Exemplary embodiments of the present invention relate to method for operating a motor vehicle diesel engine having an exhaust emission control system, comprising an oxidation catalytic converter, a particle filter, and an SCR catalytic converter, in which exhaust gas discharged from the diesel engine is passed through the oxidation catalytic converter before passing through the particle filter and the SCR catalytic converter.
PCT Patent Publication WO 99/39809 A1 describes an exhaust emission control system provided for a diesel engine, having an oxidation catalytic converter, a particle filter, and an SCR catalytic converter situated one behind the other.
In exhaust emission control systems having a particle filter and an SCR catalytic converter, it is generally problematic that, due to the operating conditions, nitrogen oxides (NOx) are emitted to the surroundings largely unconverted during thermal regeneration of a particle filter in which soot is burnt-off.
Exemplary embodiments of the present invention are directed to a method for operating a motor vehicle diesel engine having this type of exhaust emission control system, the method allowing a reduction in NOx even when a thermal particle filter regeneration with soot burn-off is performed.
For the method according to the invention, a regeneration of the particle filter with thermal soot burn-off is occasionally carried out, and during the regeneration of the particle filter, the diesel engine is operated at an air-fuel ratio having a lambda value of at least approximately 1.0, and air is added to the exhaust gas after it exits the oxidation catalytic converter and before it enters the particle filter air, so that burn-off of soot deposited on the particle filter is made possible.
The invention is based on the finding that typical diesel oxidation catalytic converters are able to convert NOx as well as reducing exhaust gas components such as carbon monoxide (CO) and hydrocarbons (HC) into harmless components, even at high exhaust gas temperatures, when the diesel engine is operated at an air-fuel ratio having a λ value of at least approximately 1.0. Catalyst formulations for diesel oxidation catalytic converters with or without oxygen storage capacity are known to those skilled in the art, so that no particular discussion of such is provided herein. In any case, coated supported catalysts, in which a metal or ceramic substrate is provided with a washcoat coating containing noble metal, are typical. Preferred noble metals are those of the platinum group, in particular platinum itself.
The SCR catalytic converter is a catalytic converter that is able to selectively and continuously reduce NOx under oxidizing conditions, i.e., at λ>1.0, using ammonia (NH3).
In this regard, the λ value characterizing the air-fuel ratio is understood to mean, as is customary, a ratio of the quantity of oxygen actually present in the combustion air-fuel mixture to the minimum quantity of oxygen theoretically required for complete combustion of the fuel. A lean air-fuel mixture having excess air therefore has a λ value of greater than one. On the other hand, a rich air-fuel mixture having excess fuel has a λ value of less than one. In the absence of oxygen sources or sinks in the exhaust gas system, the lambda value in the exhaust gas (exhaust gas λ) corresponds to the lambda value of the air-fuel mixture (combustion λ) with which the engine is operated. For simplicity, therefore, reference is made below only to a λ value, or λ for short, when differentiation is not necessary.
Due to the addition of air to the exhaust gas according to the invention before the exhaust gas enters the particle filter, the particle filter is enriched with oxygen to a greater or lesser degree, so that a soot burn-off that regenerates the particle filter may proceed, a prerequisite being a prior temperature increase to an appropriately high temperature. A temperature of the exhaust gas or of the particle filter of greater than 550° C. is typically necessary for thermal soot burn-off. A burn-off rate that is customarily sought is usually achieved only above 650° C. At such high exhaust gas temperatures, the effectiveness of an SCR catalytic converter, also provided in the exhaust emission control system, with regard to NOx conversion is already greatly reduced. This is particularly true when the SCR catalytic converter is situated relatively close to the particle filter. In the present case, however, the oxidation catalytic converter may take over this function during the particle filter regeneration. As a result, removal of NOx from the exhaust gas is achieved even during a particle filter regeneration.
A so-called wallflow filter based on silicon carbide or aluminum titanate, which may optionally have a catalytically active coating on the channel walls, is preferably used as a particle filter.
A secondary air pump may be used for supplying air to the exhaust gas between the oxidation catalytic converter and the particle filter. It is likewise possible to withdraw air from a compressed air store, or to provide air by branching off from an intake tract by means of a turbocharger. A device for variably, in particular controllably, providing quantities of air is preferred.
In one embodiment of the invention, an oxidation catalytic converter having a three-way catalyst coating is used. This involves a catalyst material, which in a narrow range around λ=1.0, is able to catalyze a conversion of NOx as well as reducing exhaust gas components such as CO and HC to harmless products. The catalyst coating typically contains platinum and/or palladium as well as rhodium, and also a material such as cerium oxide that is able to store and release oxygen. Appropriate catalyst formulations are known to those skilled in the art, in particular from applications regarding exhaust gas emission control of gasoline engines. As a result of this design, particularly effective removal of harmful exhaust gas components, NOx in particular, specifically when carrying out a particle filter regeneration, is achieved.
In another embodiment of the method, during a regeneration of the particle filter the addition of air is carried out with a mass flow such that the burn-off of soot deposited on the particle filter occurs at a predefinable rate. The soot burn-off rate is preferably determined by means of a temperature sensor that is able to detect, directly downstream from and/or in the particle filter, a quantity of heat released during the soot burn-off due to the resulting temperature development. Since the rate of the soot burn-off is a function of the oxygen partial pressure of the exhaust gas, the soot burn-off may be controlled or regulated via the added quantity of air. In this way, for example overheating of the particle filter is avoidable in that the added quantity of air, and thus the soot burn-off rate, is reduced or kept low. Supplying air which is regulated or controlled by means of a lambda sensor for achieving a desired soot burn-off rate is likewise possible. Since the air which promotes soot burn-off is supplied from an external source, engine operation which is independent from the air flow rate is made possible.
In another embodiment of the method, outside operating phases of a particle filter regeneration, the diesel engine is operated with excess air that is typical for normal diesel engine operation, and ammonia or a reducing agent, in particular an aqueous urea solution, which is capable of splitting ammonia is added to the exhaust gas before it passes through the SCR catalytic converter. NOx reduction occurs with the aid of the SCR catalytic converter. At the same time, removal of the oxidizable exhaust gas components by means of the oxidation catalytic converter is made possible.
In another embodiment of the method the exhaust gas is led through the SCR catalytic converter before and/or after passing through the particle filter. The SCR catalytic converter may be provided as a separate component upstream and/or downstream from the particle filter. In another particularly advantageous embodiment of the invention the exhaust gas is led through an SCR catalytic converter designed as a coating of the particle filter. The coating of the particle filter with an SCR catalyst material may be provided on the uncontrolled gas side or the clean gas side of the filter-active channel walls of the particle filter. It is preferred that the clean gas side is provided, at least on the downstream portion of the particle filter, with a coating containing an SCR catalyst material.
Further advantages, features, and particulars of the invention result from the following description of preferred exemplary embodiments, and with reference to the drawings. The features and feature combinations mentioned above in the description, as well as the features and feature combinations mentioned below in the description of the figures and/or shown in the figures alone, are usable not only in the particular stated combination, but also in other combinations or alone without departing from the scope of the invention.